The present invention relates to an electrophotographic copying machine and, more particularly, to a corona discharger for separating a copy paper from a photoreceptor in an electrophotographic copying machine.
An electrophotographic copying machine produces onto a photoreceptor an electrostatic latent image corresponding to a pattern image on a document such as a manuscript or book to be copied. Toner particles are electrostatically adhered to the latent image, so that the latent image becomes visible as a toner image. The toner image on the photoreceptor is transferred onto a copy paper via a transference charger.
To electrostatically transfer the toner image to the copy paper, conventionally, a corona discharge in a polarity opposed to the polarity of the toner particles placed on the photoreceptor is applied to the copy paper from the back surface of the copy paper on which no toner image is to be transferred. Since the copy paper is charged by the corona discharge, the copy paper is attracted by the photoreceptor on account of Coulomb force. Means must be provided for separating the copy paper from the photoreceptor.
By way of example, such a means may comprise a corona discharger provided for generating a corona discharge to separate the copy paper by removing the excess charges from the copy paper, thereby weakening the contact force between the photoreceptor and the copy paper. Depending on the stiffness and the weight of the copy paper, the copy paper can be separated from the photoreceptor. This is called a "corona separation method" herein.
In the conventional corona separation method, a predetermined constant voltage is applied to the separation corona discharger. The constant voltage is selected so that the separation corona discharger can output a common corona discharge voltage capable of separating all copy papers from the photoreceptor regardless of whether the copied images are all black, all white or a combination.
However, the range of such a common corona discharge voltage for separating the copied paper having any types of images from the photoreceptor is so narrow that the paper separation may fail when the surrounding conditions, the kind of the copy paper and the like, are changed so that the common corona discharge voltage is deviated from the paper separation range.